Multi-modal investigation of the bone micro- and ultrastructure, and elemental distribution in the presence of Mg-xGd screws at mid-term healing stages

Magnesium(Mg)–based alloys are becoming attractive materials for medical applications as temporary bone implants for support of fracture healing,e.g.as a suture anchor.Due to their mechanical properties and biocompatibility,they may replace titanium or stainless-steel implants,commonly used in orthopedic field.Nevertheless,patient safety has to be assured by finding a long-term balance between metal degradation,osseointegration,bone ultrastructure adaptation and element distribution in organs.In order to determine the implant behavior and its influence on bone and tissues,we investigated two Mg alloys with gadolinium contents of 5 and 10 wt percent in comparison to permanent materials titanium and polyether ether ketone.The implants were present in rat tibia for 10,20 and 32 weeks before sacrifice of the animal.Synchrotron radiation-based micro computed tomography enables the distinction of features like residual metal,degradation layer and bone structure.Additionally,X-ray diffraction and X-ray fluorescence yield information on parameters describing the bone ultrastructure and elemental composition at the bone-to-implant interface.Finally,with element specific mass spectrometry,the elements and their accumulation in the main organs and tissues are traced.The results show that Mg-xGd implants degrade in vivo under the formation of a stable degradation layer with bone remodeling similar to that of Ti after 10 weeks.No accumulation of Mg and Gd was observed in selected organs,except for the interfacial bone after 8 months of healing.Thus,we confirm that Mg-5Gd and Mg-10Gd are suitable material choices for bone implants.

Evaluating the morphology of the degradation layer of pure magnesium via 3D imaging at resolutions below 40 nm

Magnesium is attractive for the application as a temporary bone implant due to its inherent biodegradability,non-toxicity and suitable mechanical properties.The degradation process of magnesium in physiological environments is complex and is thought to be a diffusion-limited transport problem.We use a multi-scale imaging approach using micro computed tomography and transmission X-ray microscopy(TXM)at resolutions below 40 nm.Thus,we are able to evaluate the nanoporosity of the degradation layer and infer its impact on the degradation process of pure magnesium in two physiological solutions.Magnesium samples were degraded in simulated body fluid(SBF)or Dulbecco’s modified Eagle’s medium(DMEM)with 10%fetal bovine serum(FBS)for one to four weeks.TXM reveals the three-dimensional interconnected pore network within the degradation layer for both solutions.The pore network morphology and degradation layer composition are similar for all samples.By contrast,the degradation layer thickness in samples degraded in SBF was significantly higher and more inhomogeneous than in DMEM+10%FBS.Distinct features could be observed within the degradation layer of samples degraded in SBF,suggesting the formation of microgalvanic cells,which are not present in samples degraded in DMEM+10%FBS.The results suggest that the nanoporosity of the degradation layer and the resulting ion diffusion processes therein have a limited influence on the overall degradation process.This indicates that the influence of organic components on the dampening of the degradation rate by the suppression of microgalvanic degradation is much greater in the present study.